<!DOCTYPE HTML>
<html>
<head>
<title>Title</title>
<script src="jquery-2.1.1.min.js"></script>
<script src="jquery-2.1.1.min.js"></script>
<script>
	window.onload = function() {
		const PI = 3.1415;
	}
	
	$(function(){
		const foo = 3.1415;
		foo=1;
		$("#constval").html(foo);
		
		//{let letval="a";}
		//$("#letval").html(letval);
		
	});
</script>
</head>

<body>
	constval:
	<div id='constval'></div>
	letval:
	<div id='letval'></div>
</body>

</html>